Electrical power distribution systems in which generators are driven as auxiliary loads by variable speed engines have unique regulatory problems. An example is the electrical system for a multi-engine aircraft where each engine drives a generator. Engine speed is determined by aircraft operating conditions and may vary over a wide range. A constant speed drive interposed between each engine and generator converts a variable input shaft speed to a constant output shaft speed to drive the generator. A control circuit provides frequency and voltage regulation and interconnects the generators for division of the electrical load.
Abnormal system conditions may result in one or more generators being driven as motors by the other generators. This is undesirable as it results in a power drain, causes frequency shift in the system and may damage the constant speed drive. In a typical system, a mechanical overrunning clutch is provided between the constant speed drive output and the generator, allowing a generator to be driven as a motor with a minimum power drain and without damage to the constant speed drive. The clutch, however, adds initial cost to the system, is subject to wear requiring repair or replacement.